Best method to do a personal backup currently would be via RSS and save them to a local RSS reader on your home PC. You would probably be stuck with manually copyingf and pasting if something went wrong with the data here but I don't see that happening. FAQ #1 and #2. I myself would just save the RSS file and import that to a local copy of WP on one of my home servers but not everybody has Apache running at home. :)

1. It's not necessarily a new blogger. I regularly see older blogs popping up. I would guess they stay there until the stats for another blog overtakes them.

2. Minimal. The only time you'll lose all the posts is if YOU delete the blog. wp.com has previously stated they make regular and concise backups on all their servers so in the event of a major crash you'd probably lose one day's posting rather than the lot.

I think you'll find that how the stats are actually calculated is a secret - just like how search engines do not reveal their own algorithms.
As soon as you tell people how it's done certain people will exploit it to dominate and that's just unfair.
(And I haven't a clue how it's done)